If those conversations were through Siri’s ChatGPT integration, whether you can find them depends on whether you were signed in to a ChatGPT account at the time.
If you were signed in, Apple and OpenAI say those ChatGPT requests can be saved to your ChatGPT chat history. On your Mac, check System Settings > Apple Intelligence & Siri > ChatGPT and confirm which ChatGPT account is connected. Then sign in to that same account on ChatGPT and check the history.
If you were using ChatGPT through Siri without a ChatGPT account, those requests aren’t saved by OpenAI, so there isn’t a conversation history you can retrieve later.
For academic citations, I also wouldn’t cite Siri or ChatGPT as the source of factual information when the response was based on another source. If you still have the response, use it to locate and cite the original source instead.